The invention relates to a ballast water ozone injection method and system. More particularly, the invention relates to a system for injecting ozone to treat water during loading or discharge to or from the ballast tanks of a vessel or ship.
Ballast water weight is used by sea vessels to compensate for a lack of cargo weight to maintain stability when the ship cargo hold is empty or partially empty. For example in a typical transport operation, a vessel docks at a first port where it is loaded with a cargo that is transported to a second port where the cargo is unloaded. The vessel then returns to the first port where it is loaded with another cargo. Typically, the vessel travels empty from the second port back to the first port to pick up another cargo. The vessel is equipped with ballast tanks that can be filled with water to maintain the balance of the vessel when it travels empty. The ballast tank water is then discharged when cargo is loaded.
Ballast water contains species that are indigenous to the ballast tank filling location. These species are loaded into the ballast tanks along with the water. The vessel then transports ballast water to a cargo loading port where the species are discharged into the water environment along with the ballast water. The discharged species may be nonindigenous and deleterious to the discharge water environment. The nonindigenous species may cause damage to the water environment and replace benthic organisms and clear plankton communities that provide food and larvae for desirable resident native species.
In 1996, Congress passed the National Invasive Species Act (P. L. 104-332) (“NAIS”) to stem the spread of nonindigenous organisms through ballast water discharge. The act reauthorized the Great Lakes ballast management program and expanded applicability to vessels with ballast tanks. The Act requires the Secretary of Transportation to develop national guidelines to prevent the spread of organisms and their introduction into U.S. waters via ballast water of commercial vessels. The National Aquatic Invasive Species Act and the Ballast Water Management Act and other pending or to be introduced legislation regulate the treatment of salt or fresh ballast water prior to its discharge and would require that all ballast water discharged within the territorial waters of the United States (i.e. within 200 miles of the Coast or in the Great Lakes) be treated so as to kill or remove all aquatic nuisance species (i.e. bacteria, viruses, larvae, phytoplankton and zooplankton).
The water loaded into ballast tanks is a complex composition of physical, chemical and biological entities. Further, the composition of the water varies considerably from port to port, particularly in terms of biological constituents. The complexity and variation of the water makes disinfectant treatment unpredictable. Various known methods and systems for treating water may not work for treating ballast water because of a resistant life form or unexpected chemical constituency or a proposed treatment itself may degrade a local ecosystem upon discharge.
Ozonation has been found to be a safe and effective disinfectant method and system to treat ballast water for discharge into destination water environments. Rodden U.S. Pat. No. 6,125,778 first suggested an ozone ballast water treatment that included sparging into ballast water tanks. However direct tank sparging may make ozonation disinfection expensive and ineffective as not all spaces in ballast tanks may be reached. Robinson et al. U.S. Pat. No. 6,869,540 (Robinson) has suggested an in-line treatment of loading and/or unloading ballast water. The Robinson method can comprise injecting ozone into a line of water loading into a sea faring vessel prior to charging the water into a ballast tank; charging the ozone injected water into the ballast tank; and adjusting a rate of injection of the ozone into the water and adjusting the rate of water loading into the vessel to provide a target biokill of species within the water.
Robinson ozonation achieves disinfection by a sequential and combined two mechanism effect—ozonation and bromination. Ozone directly kills species by oxidation. Additionally, a reaction between ozone and naturally occurring seawater bromides results in a disinfecting bromination through the formation of hypobromous ion and hypobromous acid. The effect of the ozonation and bromination disinfecting processes has been found to be synergistic in that the combined effect is an improvement over the effects of the separate disinfectant processes.
In-line ozonation of seawater during pumping intake or discharge is more effective and more economical than in-tank treatment. However, other concerns remain. For example, off-gas can be a concern with ozone usage and some of these systems require an ozone destroying off-gas destruction device at the system exhaust end. The cost of equipment for off-gas treatment of systems of the size of a sea vessel could be prohibitive.
There is a need for an uncomplicated and cost effective system and method for direct ozonation of intake/discharge ballast water.